@article {Seyfried2015,
	title = {{The Lost City hydrothermal system: Constraints imposed by vent fluid chemistry and reaction path models on subseafloor heat and mass transfer processes}},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {163},
	year = {2015},
	pages = {59{\textendash}79},
	abstract = {Since the first reported discovery of the Lost City hydrothermal system in 2001, it was recognized that seawater alteration of ultramafic rocks plays a key role in the composition of the coexisting vent fluids. The unusually high pH and high concentrations of H2 and CH4 provide compelling evidence for this. Here we report the chemistry of hydrothermal fluids sampled from two vent structures (Beehive: \~{}90{\textendash}116 {\textdegree}C, and M6: \~{}75 {\textdegree}C) at Lost City in 2008 during cruise KNOX18RR using ROV Jason 2 and R/V Revelle assets. The vent fluid chemistry at both sites reveals considerable overlap in concentrations of dissolved gases (H2, CH4), trace elements (Cs, Rb, Li, B and Sr), and major elements (SO4, Ca, K, Na, Cl), including a surprising decrease in dissolved Cl, suggesting a common source fluid is feeding both sites. The absence of Mg and relatively high concentrations of Ca and sulfate suggest solubility control by serpentine{\textendash}diopside{\textendash}anhydrite, while trace alkali concentrations, especially Rb and Cs, are high, assuming a depleted mantle protolith. In both cases, but especially for Beehive vent fluid, the silica concentrations are well in excess of those expected for peridotite alteration and the coexistence of serpentine{\textendash}brucite at all reasonable temperatures. However, both the measured pH and silica values are in better agreement with serpentine{\textendash}diopside{\textendash}tremolite-equilibria. Geochemical modeling demonstrates that reaction of plagioclase with serpentinized peridotite can shift the chemical system away from brucite and into the tremolite stability field. This is consistent with the complex intermingling of peridotite and gabbroic bodies commonly observed within the Atlantis Massif. We speculate the existence of such plagioclase bearing peridotite may also account for the highly enriched trace alkali (Cs, Rb) concentrations in the Lost City vent fluids. Additionally, reactive transport modeling taking explicit account of temperature dependent rates of mineral dissolution and precipitation clarifies the feedback between permeability, heat loss, and changes in the dissolved Si of the vent fluids. Assuming both the Beehive and M6 vent fluids were sourced at similar subseafloor conditions (tremolite buffered at 200 {\textdegree}C), model results indicate loss of approximately 30{\%} Si upon cooling to \~{}150 {\textdegree}C during upflow. However, Si concentrations remained largely conservative with continued cooling to lower temperatures owing to unfavorable reaction kinetics. While consistent with the Beehive endmember composition, these results fail to explain the relative Si depletion in the lower temperature M6 fluids. Thus, it may be that more robust kinetic models for silicates are needed to accurately account for the mechanism and rate of silica removal in the unusually high pH of the Lost City vent fluids.},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	doi = {10.1016/j.gca.2015.04.040},
	author = {Seyfried, W E and Pester, N J and Tutolo, B M and Ding, K}
}
@article {Pester2012,
	title = {{Subseafloor phase equilibria in high-temperature hydrothermal fluids of the Lucky Strike Seamount (Mid-Atlantic Ridge, 37{\textdegree}17{\textquoteright}N)}},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {90},
	year = {2012},
	pages = {303{\textendash}322},
	abstract = {As part of an integrated study conducted at the Lucky Strike Seamount (Mid-Atlantic Ridge, 37{\textdegree}17'N) in 2008, gas-tight sampling devices were used to collect high-temperature (\~{}300~{\textdegree}C) hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression. Compared with previous observations from 1993 to 1997, the most substantial changes in vent fluid compositions are dramatically increased CO2 concentrations (\~{}5{\texttimes}, up to 133~mmol/L) and the observation of vent fluids enriched in dissolved chloride relative to seawater. Combined with an increase in $δ$13CCO2$δ$13CCO2 values by \~{}4{\textperthousand} in 2008, the elevated CO2 indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event. The additional supporting fluid chemistry is, however, similar to that of the previous sampling intervals, necessitating a reassessment of the subseafloor controls on vent fluid chemistry at Lucky Strike in the context of recently obtained geophysical data that provides the depth/extent of a steady-state magma chamber. Two-phase behavior is indicated by the chloride variability in the vent fluids; and comparison with experimental data for the associated chloride-dependent partitioning of minor/trace elements suggests the possibility of a similar source fluid for all the vent structures, while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end-members. A recently calibrated Fe/Mn geothermometer indicates minimum subseafloor equilibration temperatures of 350{\textendash}385~{\textdegree}C. However, constraints imposed by dissolved Si/Cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430{\textendash}475~{\textdegree}C and pressures near the top of the axial magma chamber (\~{}410{\textendash}480~bars), where magmatic CO2 becomes entrained in the circulating fluids. The distance between the magma chamber and the seafloor at Lucky Strike is substantially greater than at most faster spreading ridges; and we propose the resulting increased residence time in the up-flow zone leads to the re-equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction. Agreement between experimental data, thermodynamic model calculations, and dissolved concentrations of Fe, Cu, Zn, H2, and H2S in the Lucky Strike fluids reinforce the hypothesis of pH-redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical Fe/Mn geothermometer. In-situ pH measurements of the high-temperature fluids exiting the seafloor are also consistent with the model calculations.},
	keywords = {ROV Jason (Remotely Operated Vehicle)},
	doi = {10.1016/j.gca.2012.05.018},
	url = {http://dx.doi.org/10.1016/j.gca.2012.05.018},
	author = {Pester, N J and Reeves, E P and Rough, M E and Ding, K and Seewald, J S and Seyfried, W E}
}
@article {Pester2011,
	title = {{A new Fe/Mn geothermometer for hydrothermal systems: Implications for high-salinity fluids at 13{\textdegree}N on the East Pacific Rise}},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {75},
	number = {24},
	year = {2011},
	pages = {7881{\textendash}7892},
	abstract = {Field and experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal vent fluids reflects fluid{\textendash}mineral reaction at higher temperatures than those typically measured at the seafloor. To account for this and, in turn, be able to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T ({\textdegree}C)~=~331.24~+~112.41*log[Fe/Mn] has been calibrated between 350 and 450~{\textdegree}C. The apparent Fe{\textendash}Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. As an example, this methodology is used to interpret new data from 13{\textdegree}N on the East Pacific Rise, where high-temperature fluids both enriched and depleted in chloride (339{\textendash}646~mmol/kg), relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, active phase separation is clearly taking place at 13{\textdegree}N, yet the fluid Fe/Mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two-phase region. These data show the chloride-enriched fluid reflects the highest temperature and pressure (\~{}432~{\textdegree}C, 400~bars) of equilibration, consistent with circulation near the top of the inferred magma chamber. This is in agreement with the elevated CO2 concentration relative to the chloride-depleted fluids. The noted temperature derived from the Fe/Mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity. This carries the important implication that, despite being chloride-enriched relative to seawater, these fluids evolved as the vapor component of even higher salinity brine.},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	issn = {0016-7037},
	doi = {10.1016/j.gca.2011.08.043},
	url = {http://dx.doi.org/10.1016/j.gca.2011.08.043},
	author = {Pester, N J and Rough, M and Ding, K and Seyfried, W E}
}
@article {Seyfried2011,
	title = {{Vent fluid chemistry of the Rainbow Hydrothermal System (36 degrees N, MAR): Phase equilibria and in-situ pH controls on subseafloor alteration processes}},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {75},
	number = {6},
	year = {2011},
	pages = {1574{\textendash}1593},
	abstract = {The Rainbow hydrothermal field is located at 36{\textdegree}13.8'N{\textendash}33{\textdegree}54.15'W at 2300~m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10{\textendash}15 active chimneys that emit high-temperature (\~{}365~{\textdegree}C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5{\textendash}2~mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H2, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH(T,P) (400~{\textdegree}C, 500~bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH(T,P) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH(T,P) and dissolved H2 are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H2 defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite~+~magnetite~+~talc~{\textpm}~tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.},
	keywords = {ROV Jason (Remotely Operated Vehicle)},
	issn = {0016-7037},
	doi = {10.1016/j.gca.2011.01.001},
	url = {http://dx.doi.org/10.1016/j.gca.2011.01.001},
	author = {Seyfried, W E and Pester, N J and Ding, K and Rough, M}
}
@inbook {Pester2008,
	title = {{The chemistry of diffuse-flow vent fluids on the Galapagos Roft (86 degrees W) : temporal vaiability and subseafloor phase equilibria controls}},
	booktitle = {Magma to microbe : modeling hydrothermal processes at ocean spreading centers},
	series = {Geophysical Monograph},
	number = {178},
	year = {2008},
	pages = {123{\textendash}144},
	publisher = {American Geophysical Union},
	organization = {American Geophysical Union},
	address = {Washington, DC},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	isbn = {0875904432},
	doi = {10.1029/178GM07},
	url = {http://dx.doi.org/10.1029/178GM07},
	author = {Pester, N J and Butterfield, D A and Foustoukos, D I and Roe, K K and Ding, K and Shank, T M and Seyfried, W E},
	editor = {Lowell, R P}
}
